Abdulqaadir’s younger brother, Khalid, then a University of Oklahoma freshman, says fellow students beat him up in the fall of 2001 because his father had defended Moussaoui. Khalid says the F.B.I. asked him to wear a wire and inform on other Muslims, but he refused. The F.B.I. declined to comment.

Khalid dropped out of school and enlisted in the Navy’s medical corps in 2002. Yet, he says, the Navy rejected his requests for assignment to Iraq. In 2004, Navy records show, he was given a lie-detector test. Khalid says he was asked if he was a terrorist, had funded 9/11, or had trained abroad with the aim of overthrowing the U.S.

Federal agents also scrutinized his brother. According to Rob Reeves, former running backs coach at Southern Illinois, F.B.I. agents showed up at the school’s football office in 2005 to conduct a background check on Abdulqaadir.

His aspirations may have given the bureau a way to pressure his father. Menepta says that twice, in 2001 and 2004, F.B.I. interrogators implied that his level of cooperation could influence his son’s N.F.L. chances.

The N.F.L. has long maintained a close association with the F.B.I. A former high-ranking bureau official heads the league security office, which vets potential draftees, and other former agents work on his staff. Teams conduct their own checks too, often using ex-F.B.I. personnel. Former agents say that the bureau and the N.F.L. have cooperated on Super Bowl security, antidrug efforts, and solving travel visa problems for players in the now-defunct N.F.L. Europe.

If Abdulqaadir had been a premier prospect, the alleged sins of his father likely would not have been visited upon him. N.F.L. teams want the best, even with some baggage. The Minnesota Vikings drafted wide receiver Randy Moss in the first round despite a battery conviction and a positive drug test. One general manager told an agent, “If Osama bin Laden’s son ran a 4.3 40, we’d draft him.” But teams give more leeway to stars than to the interchangeable masses. Abdulqaadir, who ran a 4.6 40 for scouts in 2004, was one of dozens of running backs vying for scarce spots. Bad publicity about his father wouldn’t have been worth the headache.

Seated on a rickety telephone table, Mujahid Menepta straightens a leg, then quickly bends it. “I gave Muhammad this move, giving the leg and taking it back,” the onetime running back at St. Louis’ Soldan High says proudly.

Understandably, Abdulqaadir’s father is more comfortable talking about football than about terrorism. Once a community activist, he’s become an outcast, shunned by non-Muslims who connect him to 9/11 as well as by Muslims who suspect he’s an F.B.I. informant. He’s no longer welcome at the mosque he founded in St. Louis.

His first-floor apartment in the city’s Central West End is a grim sight: mildewy walls, tattered carpet, rain leaking into a pan. Menepta recently moved back into this dilapidated brownstone, which has been in his family for half a century, after being fired from a barbecue restaurant. In less than a year there, he rose from cook to assistant general manager, overseeing 40 employees. Menepta, who had told the restaurant’s owners about his weapons conviction but not the terrorism allegations, says his rapid promotions undid him by prompting them to search his name online. “I’m being slandered all over the internet,” he says. His employer declines to explain why Menepta was let go but says it wasn’t terrorism-related.

Muhammad and Khalid live on the brownstone’s third floor with their pit bull, Tyson. Muhammad says the pet is “a reflection” of his own relentlessness. Khalid, who was honorably discharged from the Navy last year, attends community college on the G.I. Bill. Devoted to his brother, he massages Muhammad’s sore muscles after training sessions and posted a video of his football exploits on YouTube.